As the Internet has evolved, the number of network-layer protocol addresses (2^32) has proved to be insufficient for maintaining full connectivity between the continually growing number of network devices attached to the Internet. For this reason, a new network-layer protocol, known as IPv6, has been designed to replace the currently deployed network-layer protocol, known as IPv4. The numbers 6 and 4 refer to the version numbers of the two protocols, respectively. IPv6 makes astronomically more network-layer addresses available for Internet devices (2^128, in fact). See, e.g., Internet Engineering Task Force (IETF) Request for Comments (RFC) 2373 and RFC 2460.